linux-kernel - Re: [PATCH] x86/mm: Do not verify W^X at boot up

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Message-ID: <CAHk-=wj9xBsbeoiFCBLrr3y_CdMOzNh=fD3rr_kcxYwL6vV0Jw@mail.gmail.com>
Date:   Mon, 24 Oct 2022 11:19:31 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH] x86/mm: Do not verify W^X at boot up

On Mon, Oct 24, 2022 at 8:45 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>
> Adding on the kernel command line "ftrace=function" triggered:
>
> CPA detected W^X violation: 8000000000000063 -> 0000000000000063 range:

Hmm.

The cause of this actually seems to be this

        if (likely(system_state != SYSTEM_BOOTING))
                set_memory_ro((unsigned long)trampoline, npages);
        set_memory_x((unsigned long)trampoline, npages);
        return (unsigned long)trampoline;

in create_trampoline().

And that in turn is because of commit 59566b0b622e ("x86/ftrace: Have
ftrace trampolines turn read-only at the end of system boot up"),
which in turn is because of


        if (unlikely(system_state == SYSTEM_BOOTING)) {
                text_poke_early(addr, opcode, len);
                return;
        }

in text_poke_bp(). And that, in turn, is because PeterZ ended up
special-casing this all in commit 768ae4406a5c ("x86/ftrace: Use
text_poke()")

Maybe we should just strive to get rid of all these SYSTEM_BOOTING
special cases, instead of adding yet another a new one.

There's presumably "it slows down boot" reason to avoid the full
text_poke_bp() dance, but do we really care for the "ftrace=function"
case?

                  Linus